KR20040098989A - A robust timing recovery apparatus over frequency selsctive time-varying channels and a method timing recovering - Google Patents

Abstract

PURPOSE: A timing synchronization recovery apparatus for performing a timing synchronization process under a selective time-varying channel environment of a frequency is provided to minimize a DLL tracking error by setting adaptively the most significant signal of multi-paths as a reference signal. CONSTITUTION: A multi-path estimation unit(430) estimates a multi-path of a received signal of a frequency region. A multi-path selector(440) is used for selecting the most significant signal from the estimated multi-paths and estimating a path delay value of the most significant signal. A DLL(450) is used for detecting and feeding back a symbol timing offset. A phase compensator(460) is used for compensating the sampling timing by using a fractional symbol timing offset. A window controller(420) is used for controlling an FFT window by using an integer symbol timing offset.

Description

A robust timing recovery apparatus over frequency selsctive time-varying channels and a method timing recovering

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to timing synchronization of a communication system, and more particularly, to a time synchronization recovery apparatus and a method for recovering time synchronization which can perform more robust time synchronization recovery in a frequency selective time varying channel environment.

In Europe, as a digital modulation method of a communication system, DVB-T of OFDM (Orthogonal Frequency Division Multiplexing (OFDM)) method, which can achieve a double effect of improving transmission speed per bandwidth and preventing interference, is adopted.

Such a communication system such as DVB-T requires synchronization between a transmitter and a receiver in order to perform reliable communication, and synchronization has a frequency synchronization and a timing synchronization.

Timing synchronization is divided into symbol synchronization and sampling clock synchronization.

The purpose of symbol synchronization is to find a boundary point between a guard period and a valid period in order to perform a Fast Fourier Transform (FFT) on a valid signal among OFDM symbols. On the other hand, the purpose of sampling clock synchronization is to match the sampling clock phase and sampling clock frequency of the transmitter and receiver.

Sampling clock phase offset in an OFDM communication system simply compensates in an equalizer because symbol synchronization does not cause inter symbol interference (ISI) if the symbol synchronization occurs inside a guard interval. However, because sampling clock frequency offsets cause symbol timing drift, continuous symbol timing offsets need to be estimated and compensated for.

When the estimation and compensation process of the symbol timing offset is not accurate, symbol timing jitter is generated. Symbol timing jitter leads to performance degradation of the channel estimation algorithm using general interpolation and the residual carrier frequency offset tracking algorithm using two consecutive OFDM symbols.

In order to minimize such symbol timing jitter, the Baogue Yang algorithm (Baogue Yang, Khaled Ben Letaief, Roger S. Cheng and Zhigang Cao, "Timing recovery for OFDM transmission", IEEE journal on selected areas in comm.vol. 18. No. 2000) was used to restore time synchronization using a delayed locked loop (DLL) in the frequency domain based on the signal that arrived first.

However, when the first signal arrives very weak in a frequency selective time varying channel environment, it is difficult to estimate the time path delay value of the signal. As such, when the time path delay value of the first arriving signal cannot be estimated, the DLL is operated based on the second arriving signal, thereby generating an ISI by pre-echo of the next adjacent symbol. Done.

In addition, when the reference signal of the selected DLL is subjected to deep fading for a long time, tracking of the reference signal becomes impossible, which adversely affects time synchronization.

In order to solve the above problems, there is provided a time synchronization recovery apparatus and a time synchronization recovery method capable of reliable time synchronization recovery in a frequency selective time-varying channel environment in a communication system.

1 is a schematic block diagram of a receiver having a time synchronization recovery apparatus according to the present invention;

2 is a view showing an FFT window set in an acquisition mode by a time synchronization recovery apparatus according to the present invention;

3 is a flowchart illustrating a time synchronization restoration method of a time synchronization restoration apparatus according to the present invention;

4 is a graph for explaining the performance of the time synchronization recovery apparatus according to the present invention.

Explanation of symbols on the main parts of the drawings

400: time synchronization recovery device 410: symbol timing estimation unit

420: window adjusting unit 430: multipath estimating unit

440: multi-path selection unit 450: DLL unit

451: offset detector 453: feedback unit

460: phase compensation unit

In accordance with another aspect of the present invention, there is provided a time synchronization recovery apparatus comprising: a multipath estimator for estimating a multipath for a received signal in a frequency domain; A multipath selector which selects the largest signal of the estimated multipaths and estimates a path delay value of the largest signal; And a delayed locked loop (DLL) unit for detecting and feeding back a symbol timing offset based on a path delay value of the largest signal. It includes.

Preferably, the phase compensation unit for compensating for the sampling timing based on the prime number symbol timing offset of the detected symbol timing offset; And a window adjuster that adjusts a fast fourier transform (FFT) window based on an integer multiple symbol timing offset among the detected symbol timing offsets.

The DLL unit may include an offset detector configured to calculate the symbol timing offset based on a delay value of the strongest signal; And a feedback unit which separates the calculated symbol timing offset into the decimal symbol timing offset and the integer symbol timing offset and feeds back the phase compensator and the window adjuster, respectively, wherein the offset detector is the strongest A reference signal generator configured to generate an early pilot and a late pilot using the path delay value of the signal; A first correlator and a first squarer which take a correlation value with the variance pilot of the received signal converted from the early pilot and the fast Fourier transform, and calculate a squared value thereof; A second correlator and a second squarer which take a correlation value with the variance pilot of the late pilot and the fast Fourier transform and calculate a squared value thereof; And an adder for adding each of the calculated squared values.

On the other hand, the time synchronization recovery method according to the present invention, the multi-path estimation step of estimating the multi-path for the received signal on the fast Fourier transform frequency domain; Selecting a largest signal among the estimated multipaths and estimating a path delay value of the largest signal; And a delayed locked loop (DLL) step of detecting and feeding back a symbol timing offset based on the path delay value of the largest signal.

Advantageously, said DLL step comprises: a detecting step of calculating said symbol timing offset based on a delay value of said strongest signal; And dividing the calculated symbol timing offset into the decimal symbol timing offset and the integer symbol timing offset and feeding back each of the phase compensator and the window adjuster, wherein the detecting step includes the strongest signal. A reference signal generation step of generating an early pilot and a late pilot using the path delay value of? A first calculation step of taking a correlation value with the distributed pilot of the received signal, which is the early pilot and the fast Fourier transform, and calculating a squared value thereof; A second calculation step of taking a correlation value with the distributed pilot of the late pilot and the fast Fourier transformed received signal and calculating a squared value thereof; And adding each of the calculated squared values.

Accordingly, by adaptively designating the strongest signal among the multipaths as the reference signal of the DLL, the symbol timing offset can be adaptively detected even if one of the multipaths is in a deep padding state for a long time. In addition, the IFT by the pre-echo signal can be prevented by setting a starting point of the FFT window with a certain margin inside the protection period.

Hereinafter, with reference to the drawings will be described the present invention in more detail.

1 is a schematic block diagram of a DVB-T communication system having a symbol timing recovery apparatus according to the present invention.

The DVB-T communication system includes an analog to digital converter (ADC) unit 100, a symbol timing recovery unit 400, a fast fourier transform unit 300, an equalizer 500, and a forward error corrector (FEC). Part 600 and the like.

The ADC unit 100 converts the received analog OFDM signal into a digital signal through sampling, quantization, and coding.

The time synchronization recovery apparatus 400 adjusts the FFT window start point to perform fast Fourier transform on the sampling offset generated during the sampling process of the OFDM signal by the ADC unit 100 and the valid signals among the OFDM signals.

The FFT unit 300 performs a Fast Fourier Transform on the OFDM signal based on the window start point adjusted according to the operation mode of the symbol timing recovery apparatus 400.

The equalizer 500 compensates for the distortion generated on the transmission channel with respect to the fast Fourier transformed OFDM signal output from the FFT unit 300.

The FEC unit 600 detects an error by the error detection method set for the data of the OFDM signal, and corrects the error of the detected data.

Hereinafter, the time synchronization recovery apparatus 400 according to the present invention will be described in more detail.

The time synchronization recovery apparatus 400 may include a symbol timing estimator 410, a window adjuster 420, a multipath estimator 430, a multipath selector 440, a delayed locked loop (DLL) unit 450, and The phase compensation unit 460 includes a phase compensator 460, and the symbol timing restoration process is divided into an acquisition mode ① and a tracking mode ②.

The symbol timing estimator 410 estimates the symbol timing approximately for the received OFDM signal when the operation mode of the time synchronization recovery apparatus 400 is the acquisition mode (1). In general, the received OFDM signal has a guard interval obtained by copying a symbol interval of a part of the rear part of the valid period to the beginning of the valid period in order to prevent inter symbol interference (ISI). Therefore, the boundary point between the protection section and the effective section is set approximately.

The FFT window starting point (θ _FFT ) by the boundary point of the guard interval and the effective interval that is set roughly is expressed by Equation 1 below.

Where r (k) is the k-th OFDM sample received in the time domain, N is the FFT size, L is the length of the guard interval, * is the complex conjugate, μ is the extra sample length obtained by the simulation It is preferably 20 samples.

2 is an exemplary diagram of an FFT window configured in an acquisition mode for a channel having three multipaths.

As shown, the peak value of the correlation value, which is the starting point of the protection interval, is slightly pushed back by the multipath, and the length of the protection interval L is slightly back from the boundary of the protection interval and the effective interval. Pushed back. Here, the FFT window in the acquisition mode is set by subtracting the extra sample length μ. Here, the symbol timing offset with respect to the set margin sample length μ can be compensated by the equalizer 500.

The window adjuster 420 adjusts an FFT window for performing the fast Fourier transform according to the FFT window start point set by the symbol timing estimator 410.

The multipath estimator 430 estimates the multipath for the OFDM signal in the fast Fourier transformed frequency domain in the FFT unit 300 when the operation mode of the time synchronization recovery apparatus 400 is the tracking mode (②). . That is, after extracting a scattered pilot included in an OFDM signal in a fast Fourier transformed frequency domain, the frequency response function (H ' _{n, k} ) of the subcarriers carrying each pilot is the Least Square Estimation technique. Is defined as in Equation 2 below.

Here, P ' _{n, k} is a distributed pilot carried on the k-th subcarrier of the n-th symbol in the frequency domain, and P _k is a distributed pilot known to the receiver. P _s is the index where the distributed pilot within a symbol is located. For example, in order to obtain a channel impulse response function of 1/2 interval of an OFDM sample in the time domain, M zeros are inserted and a 2M point Inverse Fast Fourier Transform (IFFT) is performed to perform an impulse response function of a channel (h " _{n, i} ), and the impulse response function (h " _{n, i} ) of the estimated channel is expressed by Equation 3 below.

Where Δ represents the spacing of adjacent scattered pilots, and H " _{n, k} represents the frequency response function of the subcarrier carrying the scattered pilot after M zeros are inserted into H ' _{n, kk} . K _{min_p} represents the position of the first subcarrier of the distributed pilot in one symbol.

The multipath selector 440 selects the strongest signal with respect to the multipath estimated by the multipath estimator 430 and estimates a path delay value thereof. That is, when the path delay value S ' _max of the strongest signal is estimated from the impulse response function h " _{n, i of the} channel obtained by Equation 3, the following Equation 4 is expressed.

The DLL unit 450 may use the path delay value S ′ _max of the strongest signal selected by the multipath selector 440 in the sampling process of the ADC unit 100 and the fast Fourier transform process of the FFT unit 300. The offset detection unit 451 detects a symbol timing offset of the generated OFDM signal, and a feedback unit 453 feeds back the detected symbol timing offset.

The offset detector 451 includes a reference signal generator 451-1, a first correlator 451-2, a first squarer 451-3, a second correlator 451-4, and a second squarer 451. -5), and an adder 451-6.

The reference signal generator 451-1 uses the path delay value S ′ _max of the strongest signal selected by the multipath selector 440 to generate an early pilot (P _e) , which is a reference signal as shown in Equation 5 below. (k)) and Late pilot (P _l (k)).

here, Represents the early or late phase of the reference signal known to the receiver, and the value is set to 0.5.

Dispersion of the early pilot _Pe (k) generated by the reference signal generator 451-1 and the fast Fourier transformed OFDM signal by the first correlator 451-2 and the first square 451-3 The correlation value for the pilot is taken to calculate the squared value.

Dispersion of the Late Pilot P _l (k) and the Fast Fourier Transformed OFDM Signal Generated by the Reference Signal Generator 451-1 by the Second Correlator 451-4 and the Second Squarer 451-5 The correlation value for the pilot is taken to calculate the squared value.

The adder 451-6 detects the symbol timing offset by adding the squared values calculated by the first and second squarers 451-3 and 451-5. That is, when the difference of the squares for each correlation value is obtained, the symbol timing offset, a (n, S ' _max ), is expressed as in Equation 6 below.

Here, E [a (n, _S'max )] is an active component and n _a (n) is a loop noise component having an average of zero. The active ingredient is represented by Equation 7 below.

Where A represents the power of the distributed pilot, S (S ' _max , ) Has a normalized S-Curve. That is, S (S ' _max , ) Is expressed by Equation 8 below.

The feedback unit 453 includes a LPF (Low Pass Filter) unit 453-1 and a NCO (Numerically Controlled Oscillator) unit 453-2. LPF section (453-1) is a symbol timing _{offset, a (n, S 'max} ) for low-pass filter and, NCO portion (453-2) in the low pass filtered symbol timing _{offset, a (n, S' max} ) For step 2, the fractional symbol timing offset Fra. And the integer symbol timing offset Int. Are separated and fed back to the phase compensator 460 and the window adjuster 420, respectively.

That is, the phase compensator 460 compensates for the sampling clock frequency offset based on the fractional symbol timing offset Fra. The window adjuster 420 adjusts the FFT window timing based on the integral symbol timing offset Int. Readjust the starting point.

Accordingly, as shown in FIG. 2, time synchronization recovery is performed by preventing drift of the start point of the FFT window set in the acquisition mode.

3 is a flowchart illustrating a time synchronization restoration method by the time synchronization restoration apparatus according to the present invention, and a time synchronization restoration method will be described in detail with reference to the flowchart.

The received OFDM signal is sampled, quantized, and coded by the ADC unit 100 and converted into a digital signal. The OFDM signal converted into a digital signal is input to the time synchronization recovery apparatus 400.

First, the time synchronization recovery apparatus 400 operates in an acquisition mode ①. That is, the symbol timing estimator 410 sets the FFT window start point θ _FFT for the received OFDM signal as shown in FIG. 2 (S311). That is, the FFT window is set by adding the guard interval length L to the correlation peak value and subtracting the extra sample length μ. The window adjuster 420 adjusts the FFT window according to the set FFT window start point.

The FFT unit 300 performs fast Fourier transform on the OFDM signal corresponding to the FFT window set in the acquisition mode (S313).

After that, the time synchronization recovery apparatus 400 operates in the tracking mode ②.

The multipath estimator 430 estimates the multipath by estimating the impulse response function h ″ _{n, i} of the channel based on the OFDM signal in the fast Fourier transformed frequency domain (S321).

The multipath selector 440 selects the strongest signal with respect to the multipath estimated by the multipath estimator 430, and selects a path delay value of the strongest signal from the impulse response function h " _{n, i} of the channel. (S _'max) the estimation (S323). Thus, the selected strongest signal is the case of deep fading is to prevent the tracking failure of a reference signal as in the prior art by selecting a new strong signal of the estimated multi-path as a reference signal have.

The DLL unit 450 detects the symbol timing offset using the path delay value S ' _max of the strongest signal selected, and feeds back to restore the time synchronization. More specifically, first, the reference signal generator 451-1 uses the path delay value S ′ _max of the strongest signal selected by the multipath selector 440 to determine an early pilot P, which is a reference signal. _e (k)) and a late pilot P _l (k) are generated (S325).

Dispersion of the early pilot _Pe (k) generated by the reference signal generator 451-1 and the fast Fourier transformed OFDM signal by the first correlator 451-2 and the first square 451-3 The correlation value for the pilot is taken to calculate the squared value.

Dispersion of the Late Pilot P _l (k) and the Fast Fourier Transformed OFDM Signal Generated by the Reference Signal Generator 451-1 by the Second Correlator 451-4 and the Second Squarer 451-5 The correlation value for the pilot is taken to calculate the squared value.

An adder (451-6) detects a first and a second squarer (451-3,451-5) squared by adding a value calculated by the symbol timing _{offset, a (n, S 'max} ) (S327).

The detected symbol timing offset, a (n, S ' _max ), is low pass filtered by the LPF unit 453-1, and the fractional symbol timing offset Fra. And integer multiple symbols by the NCO unit 453-2. The signal is separated into a timing offset Int. And fed back to the phase compensator 460 and the window adjuster 420, respectively.

Accordingly, the phase compensator 460 compensates for the sampling clock frequency offset based on the fractional symbol timing offset Fra. The window adjuster 420 applies the FFT window start point set based on the integral symbol timing offset Int. By re-adjustment, time synchronization recovery is performed (S329).

4 is a graph comparing the performance of the time synchronization recovery apparatus 400 and the time synchronization recovery apparatus using the conventional Baogue Yang algorithm according to the present invention. For example, the channel's power delay profile is set using the COST207 urban model, with a signal-to-noise ratio (SNR) of 10dB.

4 (a) is a graph showing a steady state DLL tracking error with respect to symbol timing offset when the Doppler frequency is 10 Hz. As shown, the steady state DLL tracking error of the symbol timing offset according to the present invention shows a much smaller and smoother curve.

In addition, 4 (b) indicates that when the first arriving path signal undergoes deep fading for a long time under the Doppler frequency of 5 Hz, the conventional time synchronization recovery device does not track the reference signal and causes a large tracking error. In addition, the time synchronization recovery apparatus according to the present invention can be seen that the tracking error hardly occurs.

As described above, by adaptively designating the strongest signal among the multipaths as the reference signal of the DLL, the symbol timing offset can be adaptively detected even if one of the multipaths is in a deep padding state for a long time. . In addition, the IFT by the pre-echo signal can be prevented by setting a starting point of the FFT window with a certain margin inside the protection period.

According to the present invention, the steady state DLL tracking error of the symbol timing offset is minimized by adaptively setting the strongest signal among the multipaths as a reference signal for detecting the symbol timing offset.

Second, by adjusting the starting point of the FFT window to some extent inside the protection interval, interference by the next adjacent symbol can be prevented.

Third, by setting the strongest signal among the multi-paths as the reference signal, path tracking is easier, and since the DLL S-Curve of the strongest signal has less distortion than the DLL S-Curve of the weakest signal, the tracking error of the symbol timing offset is small. .

In the above described and illustrated with respect to the preferred embodiment of the present invention, the present invention is not limited to the specific preferred embodiment described above, without departing from the gist of the invention claimed in the claims in the art Various modifications can be made by those skilled in the art, and such changes are within the scope of the claims.

Claims (20)

A multipath estimator for estimating a multipath for a received signal in a frequency domain; A multipath selector which selects the largest signal of the estimated multipaths and estimates a path delay value of the largest signal; And And a delayed locked loop (DLL) unit for detecting and feeding back a symbol timing offset based on a path delay value of the largest signal. The method of claim 1, A phase compensator configured to compensate a sampling timing based on a prime number symbol timing offset among the detected symbol timing offsets; And And a window adjusting unit for adjusting a fast fourier transform (FFT) window based on an integer multiple symbol timing offset among the detected symbol timing offsets. The method of claim 2, The DLL unit, An offset detector for calculating the symbol timing offset based on the delay value of the strongest signal; And And a feedback unit which separates the calculated symbol timing offset into the decimal symbol timing offset and the integer symbol timing offset and feeds back the phase compensator and the window adjuster, respectively. The method of claim 3, wherein The offset detection unit, A reference signal generator configured to generate an early pilot and a late pilot by using the path delay value of the strongest signal; A first correlator and a first squarer which take a correlation value with the variance pilot of the received signal converted from the early pilot and the fast Fourier transform, and calculate a squared value thereof; A second correlator and a second squarer which take a correlation value with the variance pilot of the late pilot and the fast Fourier transform and calculate a squared value thereof; And And an adder for adding each of the calculated squared values. The method of claim 1, The channel impulse response function (h " _{n, i} ) of the multipath estimated by the multipath estimator is represented by the following equation: Where Δ is the spacing of adjacent distributed pilots, H " _{n, k} is the frequency response function of the subcarrier carrying M zeros after inserting M zeros into H ' _{n, kk} , M is the number of distributed pilots in one symbol, k _{min_p} is the position of the first subcarrier of the distributed pilot in one symbol. The method of claim 5, A time delay recovery apparatus, characterized in that the path delay value (S ' _max ) of the strongest signal among the estimated multipaths is expressed by the following equation: The method of claim 6, The early pilot _Pe (k) and the late pilot P _l (k) generated based on the path delay value S ' _max of the strongest signal are represented by the following equations. Device: here, Is the early and late phases of the receiver's reference pilot, P _k is the receiver's reference pilot, and P _s is the index of the reference pilot within a symbol. The method of claim 7, wherein The symbol timing offset, a (n, _S'max ), calculated by the early pilot _Pe (k) and the late pilot P _l (k) is represented by the following equation. Device: Here, E [a (n, S ' _max )] is an active ingredient and n _a (n) is a loop noise component having an average of zero. The method of claim 8, The active ingredient, E [a (n, S ' _max )] is a time synchronization recovery device, characterized in that: Where A is the power of the distributed pilot. The method of claim 9, S (S ' _max , ) Is represented by the following equation. A multipath estimating step of estimating a multipath for a received signal on a fast Fourier transformed frequency domain; Selecting a largest signal among the estimated multipaths and estimating a path delay value of the largest signal; And And a delayed locked loop (DLL) for detecting and feeding back a symbol timing offset based on the path delay value of the largest signal. The method of claim 11, A phase compensation step of compensating sampling timing based on a prime number symbol timing offset among the detected symbol timing offsets; And And a window adjusting step of adjusting a fast fourier transform (FFT) window based on an integer multiple symbol timing offset among the detected symbol timing offsets. The method of claim 11, The DLL step, Detecting the symbol timing offset based on the delay value of the strongest signal; And And dividing the calculated symbol timing offset into the decimal symbol timing offset and the integer symbol timing offset and feeding back each of the phase compensator and the window adjuster. The method of claim 13, The detecting step, A reference signal generation step of generating an early pilot and a late pilot using the path delay value of the strongest signal; A first calculation step of taking a correlation value with the distributed pilot of the received signal, which is the early pilot and the fast Fourier transform, and calculating a squared value thereof; A second calculation step of taking a correlation value with the distributed pilot of the late pilot and the fast Fourier transformed received signal and calculating a squared value thereof; And And adding each of the calculated squared values. The method of claim 11, The channel impulse response function (h " _{n, i} ) of the multipath estimated in the multipath estimating step is represented by the following equation: Where Δ is the spacing of adjacent distributed pilots, H " _{n, k} is the frequency response function of the subcarrier carrying M zeros after inserting M zeros into H ' _{n, kk} , M is the number of distributed pilots in one symbol, k _{min_p} is the position of the first subcarrier of the distributed pilot in one symbol. The method of claim 15, The path delay value (S ' _max ) of the strongest signal among the estimated multipaths is represented by the following equation: The method of claim 16, Early pilot (P _e (k)) and late pilot (P _l (k)) generated based on the path delay value (S ' _max ) of the strongest signal are represented by the following equation. Way: here, Is the early and late phases of the receiver's reference pilot, P _k is the receiver's reference pilot, and P _s is the index of the reference pilot within a symbol. The method of claim 17, The symbol timing offset, a (n, _S'max ), calculated by the early pilot _Pe (k) and the late pilot P _l (k) is represented by the following equation. Way: Here, E [a (n, S ' _max )] is an active ingredient and n _a (n) is a loop noise component having an average of zero. The method of claim 18, The active ingredient, E [a (n, _S'max )], is represented by the following equation. Where A is the power of the distributed pilot. The method of claim 19, S (S ' _max , ) Is represented by the following equation.